This invention related to liquid chromatography and more particularly to liquid chromatographic processes for analysis for ionizing materials such as acids, bases, and amphoteric materials.
Liquid chromatography is a process used increasingly in analytical and preparative chemistry, whereby a stationary porous material is held in a chamber, such as a long column, while a mobile liquid material is passed through the porous material. In a typical case, the stationary material is a solid support, e.g. an inert powder coated with a stationary liquid agent or having a chemical agent bonded to the surface thereof. Various distinct chemical compounds contained in a carrier liquid (which usually forms the bulk of the mobile liquid) have varying affinities for the stationary liquid agent. Consequently, as the mobile liquid moves through a chromatographic column, the various chemical compounds are, as a consequence of their different affinities, delayed varying times by their contact with the stationary agent or with the chemically-bonded agent. Thus, various chemicals emerge from the column at different times and are detected individually by a refractometer, an ultra-violet light absorbtometer, or some other such analytical apparatus into which the liquid flows on leaving the chromatographic column.
Special techniques have been applied to analysis or samples containing ionic materials. In general, all of these techniques utilize the equilibrium between (a) a sample-bourne ion in the mobile phase and (b) its non-ionic form wherein it is joined to a counter ion to achieve a retarding of the movement of the sample-bourne ion as it moves down the column.
The more common of these techniques is that known as "ion exchange." In that process, the stationary phase is an ionic material, which may be called an ion exchange resin, characterized by the presence of a counter ion which will readily exchange with an ion of a compound in the sample being ionized. The conditions are generally selected so that, theoretically, there is to be no net change in the condition of the ion exchange packing during the analytical procedure. In most practical situations, however, there is a substantial modification of the exchange resin over a period of time.
Some problems with such ion exchange columns of the type described include: (1) A poorer resolution of ionic components within a sample than is desired in many cases; (2) An undesirably short life because the packing tends to become contaminated and damaged by the buffered solutions required to regenerate its ion exchange potential; (3) Ion exchange columns, in order to maximize their limited analytical potential, often require the use of relatively expensive gradient elution apparatus and procedures. The second-listed disadvantage often can be remedied by preliminary purification of the sample; but, that, too, is often an expensive, time-consuming operation.
It is noted that a substantial amount of art relating to ion-exchange processes is disclosed in U.S. Pat. Nos. 3,915,642 and 3,918,906.
Another chromatographic process has been used in analysis of ionic compounds. It utilizes that species of chromatographic process known as liquid-liquid partition chromatography. This type of process utilizes a thin liquid coating (usually immobilized on fine particles of brick dust, silica, or the like) as the stationary phase. Such stationary phase materials are to be distinguished from those wherein a chemical bond (such as a covalent bond) is formed between the solid support and the active stationary phase. Liquid-partition processes, looked at schematically, do not differ much from the ion exchange-type process. In each case, the counter ion is carried on the packing. Once a major aspect of liquid chromatography, liquid-liquid partition process have been largely discarded, because drawbacks inherent in the system make it impossible to keep pace with the requirements of, and the refinements in, other areas of, liquid chromatography. E.g., it was difficult to achieve an adequate permanent fixation of the stationary phase to the base particles. Nevertheless, some investigators have placed an aqueous solution of a counter ion on the stationary phase. They then placed a sample of the material to be analyzed in a non-polar mobile phase which was passed through the counter-ion-bearing bed. There are a number of problems associated with such a process in addition to these problems inherently associated with liquid-liquid partition processes. For example, the sample had to be solubilized in a non-polar liquid (e.g., hexane) and this greatly reduced the potential use of the process.
In addition to the processes discussed above, there are procedures wherein an ester, a sulfonamide or some other compound was first formed by reaction of acid or base for which an analysis was desired: this derivative compound would be dissolved in a carrier liquid and subjected to liquid chromatography thereby facilitating the analysis of the acid or base.
It has remained a problem to achieve a widely-applicable process for analysis of ionic materials by liquid chromatography.